This invention is directed to threshold switching devices which exhibit negative differential resistance (NDR), and to certain ceramic threshold switching devices which are useful as thin film memory devices.
Devices which exhibit threshold switching, and metal oxide devices which exhibit threshold switching with negative differential resistance (NDR) are known in the art. The switching and negative differential resistance (NDR) characteristics of silicon oxide films has also been described in the literature.
The use of thin film silica coatings derived from hydrogen silsesquioxane resin to provide protection and electrical insulation is not new, but the use of those coatings to form switching devices which are useful as thin film memory devices is believed to be novel.
Coatings and switching devices formed by depositing a thin hydrogen silsesquioxane derived silicon dioxide film between at least two electrodes and applying a voltage above a threshold voltage across the electrodes, are described in detail in a prior copending patent application U.S. Ser. No. 07/694721 filed May 2, 1991, and entitled "Threshold Switching Devices". The present invention involves these and similar devices and their utility in thin film memory applications.
Memory arrays containing thin film memory devices according to the present invention offer the advantage that information can be retained without a holding voltage, i.e. true non-volatile memory. Further, it is possible to employ simplified metallization patterns because there is only one active element plus one passive element which is the load resistor. In addition, simplified address and read/write line patterns can be used.
State of the art static random access memory (RAM) cell arrangements typically require a supply voltage connection for memory retention; two cross-coupled transistors for the storage of 0 and 1, and 1 and 0; two load resistors; two access transistors controlled by the same wordline; and two bit lines, one for 0 and the other for 1. In contrast, the static RAM cell of the present invention containing the hydrogen silsesquioxane resin-derived silica switching cell offers the benefits that no permanent power supply is required, and that there is no power dissipated in the memory cell. Further, only one storage cell and one load resistor are necessary. In addition, only one access transistor is needed for WORD SELECT; and only one bit line is necessary for BIT SELECT for writing with voltage pulse and for reading data.